More precisely, said laser source is of the known type, comprising:                an active element comprising a doped matrix capable of absorbing a pump beam in order to amplify at least one beam of laser radiation;        a pumping system, comprising pump (laser) diodes capable of emitting a pump beam penetrating into said rod;        an optical transport system for directing the pump beam emitted by said pumping system into said active element so as to pump said active element; and        an optical cavity for extracting said laser radiation.        
In this laser source to which the present invention relates, the pumping may be in a longitudinal, transverse, semi-transverse or other orientation relative to said active element.
It is known that, to be effective, the pump beam must be spectrally tuned to the absorption spectrum of the active element in such a way that said pump beam is absorbed and transfers its energy into the rare-earth ions that dope said active element.
It is also known that pump laser diodes have an emission spectrum, generally a few nanometers in width, which is shifted by 0.25 to 0.3 nanometers per degree when the temperature of said pump diodes changes.
To ensure satisfactory conformity of the wavelength of the pump beam (output by the pump diodes) with the absorption spectrum of the active medium, it is known to mount said diodes on Peltier modules, the function of which is to stabilize their temperature with an accuracy of better than 0.5° C. so that wavelength centering to within 0.2 nm is ensured.
However, especially in the case of military applications, compactness, consumption and implementation rapidity parameters assume a particular importance. Thus, the use of Peltier modules, which involves considerable consumption and requires a stabilization time of the order of one minute, is a retarding factor on the use of diode-pumped laser sources in compact systems. The same applies in the case of other active systems for stabilizing the temperature of the diodes. Thus, the technology always employed at the present time, for example for terrestrial laser designators, is that of a flash pump, which is not very efficient and is bulky.
To try to remedy this problem, it is necessary:                either to increase the tolerance of the active medium to the wavelength drift, which is proposed for example by Patent FR-2 803 697, for which the pump beam is guided in order to pass several times through the active medium;        or to passively stabilize the emission of the wavelength of the pump diodes, as proposed for example in patent application US-2005/0018743, which describes the use of a system including one or more VBGs (Volume Bragg Gratings) so as to partition one or more of the emission characteristics of the laser.        
However, the above solutions only allow an insensitivity over 3 to 10 nanometers to be obtained, corresponding to a temperature drift of the diodes of 15 to 40° C. Such a thermal insensitivity range is largely insufficient for using the pumping system, for example in a terrestrial laser designator, between −40° C. and 70° C.